1. Field of the Invention
This invention relates to an improved infrared seeker assembly and particularly to one having a novel dewar coldfinger construction.
2. Description of the Prior Art
Infrared detection systems are often used in conjunction with munitions and night vision systems for sensing electromagnetic radiation in the range of wavelength of one to fifteen micrometers. Because of many of such systems have detector arrays which are most sensitive when operated at about 80.degree. K., a cooling system is required to produce and maintain the required low operating temperatures. Typically, such cooling systems either take the form of a cryostat using the Joule-Thompson effect, or a Stirling cycle cryoengine. The cooling systems are used in conjunction with a dewar in which the infrared detector is placed. The dewar is evacuated to remove gasses which would otherwise occupy the area surrounding the detector so that potential heat loss through convection and conduction is minimized. The evacuated dewar also prevents moisture from condensing on the detector. The dewar has a cylindrical tube referred to as a "coldfinger" having an end which is cooled and supports the detector and related components.
A number of design constraints affect the design of the dewar coldfinger. Since the coldfinger is a cantilever supported cylinder, it must have sufficient bending stiffness to control deflection of the infrared detector. Such requirements become particularly significant when the infrared seeker assembly is used as part of munitions subjected to intense vibration and high levels of boost-phase acceleration. Another significant design parameter is the extent to which heat is transferred from the warm end of the coldfinger cylinder to its cold end. Reductions in heat transfer rate allow the capacity of the cryogenic cooling system to be reduced. Unfortunately, reduction in the cross-sectional solid area of the coldfinger tube for reducing heat transfer adversely affects its bending stiffness for a given material and tube diameter. Another design consideration is the cool-down rate for the cold end components of the coldfinger. Since infrared seekers are often used in expendable munitions which must acquire a target soon after (or before) their launch, cool-down time becomes a critical consideration for some applications.
Prior art dewar coldfingers have been formed from various materials. Glass has been used since it has low thermal conductivity, but, unfortunately, does not yield low conductance coldfingers because it is too fragile when it is made in decreased thicknesses desired to minimize conduction heat loads. Examples of such glass dewar coldfingers are described by U.S. Pat. Nos. 3,851,173 and 3,719,990. Other prior art coldfingers have been made form various metals, for example, as described by U.S. Pat. No. 4,528,449. Metals are less fragile than glass but still cannot be made thin enough to obtain lower heat loads than glass due to their higher thermal conductivity. Plastic materials have also been used for non-evacuated detector units and have obtained low heat loads. However, due to their porosity, plastic coldfingers have not been used in vacuum type dewars.